MANITOU ISLANDS COMPLEX

Manitou Islands Complex, Ontario Canada

a Carbonatite volcanoes with magma composed primarily of rock rich in silica and contain niobium.(Eyles 2013)

The red circle identifies the position of the Manitou Islands within Lake Nipissing, Ontario. GOOGLE

The Manitou Islands Complex is an elliptical ring structure about 2 miles long and up to 1.7 miles across. Most of the complex lies beneath Lake Nipissing and only fenitic rocks of the outer part of the complex, containing a few carbonatite intrusions, are exposed on the five islands of the Manitou group. (courtesy Google)

The Manitou Islands complex from my bird GOZooM from approximately 2000′ above the lake.

Unlike most lakes in Ontario, Lake Nipissing contains two volcanic pipes, one of which is the Manitou Islands (The Callander Bay complex is the other one). The volcanic pipes formed by the violent, supersonic eruption of deep-origin volcanoes. Lake Nipissing lies in the Ottawa-Bonnechere Graben, a Mesozoic rift valley that formed 175 million years ago. (Wikipedia)

Aeromagnetic map of the Manitou Island complex (from ODM-GSC 1965c) Abstract, (Rowe 1954) Two concentric fenitic zones comprise the outer part of the complex : 1 ) an outer zone of quartz fenite as much as 400 feet wide ; and 2 ) an inner zone of aegirine-potassic feldspar fenite as much as 1,500 feet wide .

Quartz fenite is foliated due to crudely aligned , lenticular concentrations of aegirine and to elongate feldspar grains , but the aegirine-potassic feldspar fenite is mainly massive with only locally aligned aegirine – rich streaks. Generally conformable lenticular intrusions of carbonatite are present locally in the aegirine-potassic feldspar fenite, and near these intrusions , the fenite is brecciated and carbonatized , and xenoliths of carbonatized fenite are present in the adjacent carbonatite. The carbonatite is essentially a fine – to coarse-grained calcite rock and is massive to slightly foliated with streaks rich in aegirine and variable amounts of sodic amphibole , biotite , magnetite , apatite , and , locally , pyrochlore and pyrite . The composition and structure of the inner part of the complex is poorly known , but diamond drilling indicates that the inner western margin of the fenite aureole is bounded by altered , massive , coarse-grained t o pegmatitic pyroxene-ric h rocks younger than the fenites and ranging i n composition from alkalic pyroxenite to alkalic mafic syenite. Perthitic potassic feldspar and aegirine (partly altered to fine-grained carbonate) , white mica , and chlorite are the major minerals in these rocks , but altered nepheline is present in some syenitic varieties together with interstitial plagioclase . Other minerals present in variable amounts are calcite , biotite , sodic amphibole , apatite, and iron-titanium oxide minerals ; accessory minerals are pyrite , monazite , fluorite , and pyro ¬ chlore (largely altered to a fine-grained aggregate of carbonate) , hematite , and chlorite . North o f Newman Island , altered feldspathi c rocks form the innermost phase of the complex. Abundant lamprophyre dikes , minor altered basaltic dikes , and a few phonolitic dikes cut across the fenitic rocks in the outer part of the complex .

The Manitou Islands Complex is anelliptical ring structure about 2 miles long and up to 1.7 miles across. Most of the complex lies beneath Lake Nipissing and only fenitic rocks of the outer part of the complex, containing a few carbonatite intrusions, are exposed on the five islands of the Manitou group. Ordovician sedimentary rocks overlie rocks of the complex on all five islands. Concentrations of pyrochlore present in the fenitic rocks and carbonatite were explored extensively by diamond drilling and underground workings in the 1950s; the following summary of the complex is taken largely from Rowe (1954 ; 1958). Two concentric fenitic zones comprise the outer part of the complex :

1) an outer zone of quartz fenite as much as 400 feet wide ; and

2 ) an inner zone of aegirine-potassic feldspar fenite as much as 1,500 feet wide.

Quartz fenite is foliated due to crudely aligned , lenticular concentrations of aegirine and to elongate feldspar grains, but the aegirine-potassic feldspar fenite is mainly massive with only locally aligned aegirine rich streaks. Generally conformable lenticular intrusions of carbonatite are present locally in the aegirine-potassic feldspar fenite , and near these intrusions, the fenite is brecciated and carbonatized, and xenoliths of carbonatized fenite are present in the adjacent carbonatite. The carbonatite is essentially a fine-to coarse-grained calcite rock and is massive to slightly foliated with streaks rich in a egirine and variable amounts of sodic amphibole, biotite , magnetite, apatite , and, locally, pyrochlore and pyrite. The composition and structure of the inner part of the complex is poorly known, but diamond drilling indicates that the inner western margin of the fenite aureole is bounded by altered, massive, coarse-grained to pegmatitic pyroxene – rich rocks younger than the fenites and ranging in composition from alkalic pyroxenite to alkalic mafic syenite. Perthitic potassic feldspar and aegirine ( partly altered to fine – grained carbonate), white mica, and chlorite are the major minerals in these rocks, but altered nepheline is present in some syenitic varieties to gether with interstitial plagioclase. Other minerals present invariable amounts are calcite, biotite , sodic amphibole , apatite, and iron – titanium oxide minerals ; accessory minerals are pyrite , monazite , fluorite , and pyrochlore ( largely altered to a fine – grained aggregate of carbonate), hematite , and chlorite. North of Newman Island, altered feldspathic rocks form the innermost phase of the complex. Abundant lamprophyre dikes, minor altered basaltic dikes, and a few phonolitic dikes cut across the fenitic rocks in th e outer part of the complex. (Lumbers 1971)